Travelling wave amplifier



Dec. 5, 1962 T. G. LEE ETAL TRAVELLING WAVE AMPLIFIER 3 Sheets-Sheet l mm mm Filed March 17. 1958 7Z=PPY 6 LEE Awaerdmvews INVENTORS Arrow/5y Nu m. n

United States Patent 3,070,725 TRAVELLING WAVE AMPLIFIER Terry G. Lee, Palo Alto, and Albert John Sievers, San Bruno, Calif., assignors to Eitel-McCullongh, Inc., San Bruno, Calih, a corporation of California Filed Mar. 17, 1958, Ser. No. 722,102 16 Claims. (Cl. 315-35) This invention relates generally to a high frequency amplifier and more particularly to a high frequency amplifier of the travelling wave type.

As is well, known, travelling wave tube amplifiers include an electron gun which serves to project an electron beam and a collector disposed to intercept said beam. A slow wave structure which supports the electromagnetic waves to be amplified is disposed whereby the electromagnetic waves are coupled to the electron beam. The slow wave structure is such that the electromagnetic energy applied thereto has an axial component of velocity which is equal to the velocity of the electron beam. The

beam and electromagnetic waves carried by the slow wave structure interact. The interaction serves to bunch or ing.

modulate the electron beam. Energy is transferred from the beam to the electromagnetic wave and the wave is amplified.

One type of travelling wave tube amplifier employs a slow wave structure whichis wound in the form'of a helix. The electromagnetic wave is applied to the conductor forming the helix and travels with the speed of electromagnetic waves along the conductor. However, the axial component of velocity is dependent upon the pitch of the helix. This is adjusted whereby the axial velocity of the wave is substantially equal to the axial velocity of the electron stream.

Some of the electrons from the beam impinge upon the helical conductor. When they impinge upon the conductor, they give up energy in the form of heat. This tends to raise the temperature of the slow wave structure. Further, the electromagnetic wave heats the conductor. As it is amplified, the power carried by the convolutions of the helix is increased and this portion of the helix becomes hotter. ,As a result, the power handling ability of the travelling wave tube is limited by the amount of heat that may be dissipated from the helix, particularly the output end of the helix.

Another difliculty which has been experienced is that of coupling energy onto and off of the helical conductor. Various means have been employed in the past, for example, wave guide couplings, helix to helix and stripline to coaxial. In all such couplings it has been relatively diflicult to obtain eflicient coupling between the input and output of the helix and the associated microwave circuits.

It is a general object of the present invention to provide an improved travelling wave tube amplifier.

It is another object of the present invention to provide a travelling wave tube amplifier capable of handling relatively large amounts of power.

It is another object of the present invention to provide a travelling wave amplifier of novel metal ceramic construction.

7 It is another object of the present invention to provide an improved coupling which forms part of the envelope and which may be used in conjunction with wave guides or coaxial lines of associated networks. An associated object is to provide means for joining the coupling to wave guides and coaxial lines.

It is still another object of the present invention to pro vide a travelling wave tube in which a helix is suspended within the envelope by means which have a relatively high heat conduction to the tube envelope.

These and other objectsof the invention will become I more clearly apparent from the following description ice when taken in conjunction with the accompanying draw- Referring to the drawing: 7

FIGURE 1 is a side elevational view in section of a travelling wave tube in accordance with the invention;

FIGURE 2 is an enlarged view of the input end of the travelling wave tube;

FIGURE 3 is a sectional view taken 'along the line 3-3 of FIGURE 2;

FIGURE 4 is an enlargedview of the microwave coupling connected to a coaxial line;

FIGURE 5 is a sectional view taken along the line 5-5 of FIGURE 4;

FIGURE 6 is an enlarged view of the novel microwave I coupling connected to a wave guide; and

FIGURE 7 is a sectional view taken along the line 7-7 of FIGURE 6.

Referring to FIGURE 1, the travelling wave tube illustrated includes an electron gun 11 which serves to project an electron beam past the helical conductor 12. The beam is intercepted by the collector 13. Electromagnetic waves are coupled onto the helical conductor 12 by means of a coupler 14 which forms a portion of the evacuated envelope. The amplified electromagnetic waves are coupled ofl. the helical conductor at the collector end of the tube by means of a coupler 16 which also forms a portion of the evacuated envelope.

The electron gun includes an indirectly heated cathode 17 and a metallic accelerating anode 18 which accelerates the electrons and for-ms the electron beam. The electron gun illustrated is of metal-ceramic construction with the cathode assembly insulated from the anode by means of a ceramic ring 19. The ring 19 is suitably brazed at one end to the flange 21 which is brazed to the cathode structure and at its other end to the member 22 which, in turn, is brazed to the flange 23 secured to the anode structure. The metal-ceramic brazes are in accordance with well known techniques.

The main portion of the travelling wave tube envelope comprises an elongated metallic cylinder 24, for example, molybdenum, which surrounds the helix (slow wave structure) 12. The metallic cylinder 24 is held coaxial with respect to the anode 18 by means of a ceramic cylinder 26 which is sandwiched between the cylinder 24 and anode 18. A flange 27 is secured to the envelope portion, as for example, by brazing. A flange 28 is suitably brazed to the adjacent end of the anode 18. A cylindrical ceramic ring 29 is suitably brazed to the outwardly extending flange 28. A flange 31 is brazed to the other end of the ceramic ring. Thus, the anode 18 constitutes a portion of the evacuated envelope and carries the assembly comprising flanges 28 and 31, one at each end ofthe ring 29. When the anode is placed within the cylinder 24, the periphery of flanges 27 and 31 are joined to form the vacuum envelope.

The collector assembly 13 includes a fly-trap collector 33 forming an integral partof the envelope and which carries the exhaust tabulation 34. The collector 33 is cooled by a suitable fluid. Thus, it is surrounded by a jacket 36 which serves to receive the cooling fluid supplied through the tube 37. An outer jacket 38 surrounds the jacket 36 and is connected to the output tube 39. Thus, the cooling fluid flows inwardly through the inner jacket over the collector and out through the outer jacket.

The metallic collector assembly is carried at the collector end of the envelope portion 24 by means of a metal to ceramic assembly 40. The assembly illustrated includes an outwardly extending flange 41 suitably brazed to the adjacent end of the metallic envelope 24. A ring-like metallic element 42 has one end brazed to the flange 41 and its other end brazed to the ceramic ring Patented Dec. 25, 1962 43. An outwardly extending flange 44 is brazed to the collector and has its outer edge brazed to the ceramic ring 46. A ring-like element 47 is carried between and brazed to the ceramic rings 43 and 46. The element 47 provides electrical connection to the getter, to be presently described, and shields the ceramic ring 46 against deposit of getter material and from electron bombardment. A getter is formed'by disposing a plurality of ribbon-like barium elements 48 between the rings 42 and 47. Electric current may be supplied to the elements 48 by connecting to rings 42 and 47. In the absence of a getter, the ceramic rings 43 and 46 may be formed as a single ceramic ring which extends between the ring 42 and outwardly extending flange 44, however, the ceramic ring is then subjected to electron bombardment, and the deposit of barium may form a conductive film which shorts the element 42 to the flange 44.

An electromagnetic coil 51 surrounds the elongated envelope and provides an axial magnetic field which maintains the electron beam in focus as it travels from the electron gun 11 to the collector 13. The electromagnetic assembly 51 supports the traveling wave tube at the collector at one end and at the electron gun at the other end. Means may be provided for aligning the tube within the magnetic structure. For example, adjustable screws 55 may engage the electron gun and collector. A split ring 52 closes the magnetic circuit at the collector end. It includes a shoulder 53 which serves to receive the adjacent portion of the electromagnetic assembly. A split ring 54 is carried at the gun end of the tube .and closes the magnetic circuit at that end. The split ring includes a shoulder 50 which receives the adjacent portion of the electromagnetic assembly 51. End rings 52 and 54 have oversized central apertures to permit slight adjustment of the tube therein by screws 55 to obtain perfect alignment.

The helical conductor (helix) 12 is carried within the metallic envelope 24 by circumferentially spaced insulating rods 56 (FIGURES l, 2 and 3). Suitable locating means 57,surround the rods and serve to maintain the same circumferentially spaced. The locating means may, for example, comprise locating sleeves 57 which extend substantially the entire length of the rods as shown in FIGURES 1 and 2 and which have a configuration of the type illustrated to receive the rods 56. One or more wedging members 58 are placed between the metallic envelope 24 and the locating means 57 and serve to wedge the various elements to obtain a tight fit between the same. The rods 56 are made of suitable electrical insulating material which is capable of withstanding relatively high temperatures; for example, they may comprise sapphire rods. The rods 58 may be made of metal, for example, stainless steel. The members 57 may be metallic members, for example, they may be made of molybdenum or the like. With the tube assembled, the sapphire rods provide a relatively efficient electrical insulation between the metal envelope 24 and the helical conductor. However, the sapphire rods and the competent contact between the various parts provide a relatively good heat conduction path to the envelope portion whereby heat is conducted from the helix 12 to the envelope portion. The envelope may be cooled by radiation and convection; forced air cooling may be employed, or a jacket may be provided through which a coolant is circulated.

It is desirable to prevent reflections of electromagnetic waves from the output to the input since they might cause oscillations. Thus, attenuating material is located near the middle of the helical conductor. The material is applied over a predetermined small length of the sapphire rods as indicated at 61.

The couplers 14 and 16 provide means for coupling energy onto and oil the helix. Since the couplers are identical, only the input coupler is described in detail.

The end of the helix is attached to a conductor 60 which extends outwardly through an opening or aperture formed in the metallic envelope portion 24. The end is secured to a sealing plate 62, extending partly beyond the plate for purposes to be hereinafter described. The portion adjacent the plate is tapered to reduce reflection of electromagnetic waves. It is apparent, of course, that other configurations may be employed to give a smooth transition. A hollow cylindrical metal member 63 is suitably secured, for example, by brazing one of its ends in the aperture formed in the elongated envelope portion 24 and extends radially outwardly therefrom. A flange 64 is formed at the upper or end of the cylindrical portion 63, remote from the envelope portion 24. The flange '64 serves to receive a tubular ceramic window 66. The window 66 is hermetically brazed between the sealing plate 62 and flange 64. The window portion 66 permits electromagnetic energy to travel outwardly from the evacuated envelope. As is well known, a window must have a predetermined area to transmit electromagnetic energy. The cylindrical window provides suflicient area without introducing unwanted reflections. A downwardly extending cup-shaped member 67 is secured to the upper portion of the cylindrical member 63. The cup-shaped portion 67 flares slightly outwardly for purposes to be presently described.

Each of the couplers 14 and 16 is adapted to be connected either to a coaxial transmission line or to a wave guide which then serves to transmit the energy to associated apparatus. Referring to FIGURES 4 and 5, the coupler is shown connected to a coaxial transmission line 71. The transmission line 71 includes an outer conductor 72 and an inner conductor 73 which is held coaxial within the outer conductor by means of dielectric sleeves 74 spaced along its length. The outer conductor is suitably secured to a cap 76. The cap 76 is adapted to be connected to a member 77 by means of screws 78. The member 77 is adapted to partially surround the travelling wave tube envelope 24. A mating member 80 is adapted to be secured thereto to clamp the tube envelope 24 therebetween. The opening 79 formed in the member 77 is adapted to tightly fit over the downwardly extending cup-shaped member 67. As previously described, the member 67 flares slightly outwardly. Thus, a tight fit is obtained between the opening 79 of the member 77 and the cup-shaped member 67. The cap 76 and member 77 form an enclosure 81 which surrounds the window 66. The conductor 60 is allowed to extend upwardly through the sealing plate 62. A conductor 82 is adapted to engage the outwardly extending portion of the conductor 60 and is suitably attached at its other end to the inner conductor 73 of the coaxial line. The enclosure 81 serves to form a suitable transition between the coupler and the associated coaxial line whereby a minimum of reflections is set up in coupling energy onto or off of the helical conductor of the travelling wave tube.

The coupler may also be used in conjunction with a wave guide. Referring to FIGURES 6 and 7, a wave guide is shown suitably attached to the coupler. The wave guide 86 is of the ridged type to provide transmission of the energy. The guide comprises a lower portion 87 and a top plate 88 which is suitably attached thereto. The lower portion is provided with an opening 89 which is adapted to tightly fit over the downwardly extending member 67. The top plate 88 carries a contact means 91 which is urged downwardly against the sealing plate 62 by means of a screw 92 rotatably attached thereto. The end of the wave guide is suitably closed by a plate 93. A clamping member 94 is suitably secured to the lower portion 87 and serves to hold the wave guide on the coupler. The wave guide then pro vides means for coupling energy onto or olf of the travelling wave tube helix.

A travelling wave tube is provided in which the helical conductor (slow wave structure) is efliciently-cooled by providing a heat conduction path having a relatively high conductivity between the conductor and the metallic cylindrical envelope portion surrounding the same. Novel coupling means are provided for coupling energy onto and off of the helix. used in conjunction with either wave guide or coaxial lines. The tube assembly is of metal-ceramic construction which can resist relatively high temperature and which is strong and rigid. The gun, envelope and collector are electrically insulated, one from the other, whereby ditferent voltages may be applied to the same.

We claim:

1. A travelling wave tube including an elongated evacuated envelope, an electron gun disposed at one end of said envelope serving to projectan electron beam, a collector disposed at the other end of said envelope and serving to intercept said beam, a helical conductor disposed coaxially within the envelope, and means serving to hold said helical conductor coaxially within said envelope, said means comprising circumferentially spaced electrical insulating rods having their axes parallel to the longitudinal axis of the helical conductor, thermally conductive locating means surrounding said rods and resiliently maintaining the same at predetermined circumferential spacing, and thermally conductive means interposed between said locating means and saidelongated envelope serving to provide a tight mechanical fit between the rods, the locating means and the elongated envelope whereby the helical conductor has a relatively high heat conduction path to the envelope.

2. A travelling wave tube including an elongated evacuated envelope, an electron gun disposed at one end of said envelope and serving to project an electron beam, a collector disposed at the other end of said envelope and serving to intercept said beam, a helical conductor disposed coaxiallywithin the envelope, said envelope having an elongated cylindrical envelope portion adjacent to said helical conductor which has a relatively high thermal conductivity, and means serving to hold said helical conductor within said envelope portion, said means comprising circumferentially spaced electrical insulating rods having their axes parallel to the longitudinal axis of the helical conductor, locating means surrounding said rods over a substantial portion of their length and maintaining the same at predetermined circumferential spacing, and means wedgingly interposed between said locating means and said envelope portion serving to provide a tight mechanical fit between the rods, the locating means and the envelope portion where by the helical conductor has a relatively high heat conduction path to the envelope portion.

3. A travelling wave tube including an elongated evacuated envelope, 'an electron gun including a tubular metallic anode disposed at one end of said envelope and serving to project an electron beam, a collector disposed at the other end of said envelope and serving to intercept said beam, a helical conductor disposed coaxially within the envelope, said'envelope portion adjacent to the helical conductor being a tubular metallic portion, means serving to hold said helical conductor coaxially within said metallic envelope portion, an outwardly extending flange member carried by said anode portion, an outwardly extending flange portion carried by said metallic envelope portion adjacent to said anode, and a ceramic ring secured between said flanges and serving to form a portion of the envelope and electrically insulate the electron gun from the tubular metallic portion.

4. A travelling wave tube including an elongated evacu- I ated envelope, an electron gun including a metallic anode disposed at one end of said envelope and serving to project an electron beam, a metallic collector disposed at the other end of said envelope and serving to intercept said beam, a helical conductor disposed coaxially within The coupling means may be said envelope, said envelope portion adjacent said helical conductor being a tubular metallic portion, means serving said tubular metallic envelope portion, and a ceramic ring hermetically secured between said flanges and forming part of the elongated envelope, said ceramic ring serving to electrically insulate the tubular metallic envelope portion from the electron gun, an outwardly extending flange secured to the other end of said tubular metallic portion, an outwardly extending metallic flange secured to the metallic collector, and a ceramic ring secured between said last mentioned flange and serving to form a part of the envelope, said ceramic ring serving to electrically isolate the collector from the envelope portion.

5. A travelling wave tube including an elongated evacuated envelope, an electron gun disposed at one end of said envelope and serving to form an electron beam, a

collector disposed at the other end of said envelope and serving to intercept said beam, a helical conductor disposed coaxially within said envelope, said cylindrical envelope including a tubular metallic portion adjacent said helical conductor apertured adjacent opposite ends thereof, means serving to hold said helical conductor coaxially within said envelope portion, and coupling means operatively associated with said apertures for coupling electrical energy onto and off the helical conductor, each said coupling means comprising a hollow cylindrical metallic portion at one end forming a seal with the associated aperture in the elongated envelope portion, a tubular ceramic cylinder forming a window and having its inner end bonded to the other end of said cylindrical portion, a sealing plate carried on the outer end of the ceramic cylinder and forming a hermetic seal therewith, and a conductor secured to said sealing plate and connected to the adjacent end of the helical conductor.

6. A travelling wave tube including an elongated evacuated envelope, an electron gun disposed at one end of said envelope and serving to form an electron beam, a collector disposed at the other end of said envelope and serving to intercept said beam, a helical conductor disposed coaxially within said envelope, said cylindrical envelope including a tubular metallic portion adjacent said helical conductor, means serving to hold said helical conductor coaxially within said envelope portion in electrically insulating relation thereto, and coupling means for coupling electrical energy onto and off the helical conductor, each said coupling means comprising a hollow cylindrical metallic portion having its axis angularly disposed to the axis of said helical conductor and at one end forming a seal with the elongated envelope portion, a tubular ceramic cylinder forming a window and coax- I ially mounted at its inner end on the other end of said cylindrical portion, a sealing plate carried at the outer end of the ceramic cylinder and forming a seal therewith, the adjacent end of the helical conductor being electri cally connected to said sealing plate.

7. A travelling wave tube including an elongated evacuated envelope, an electron gun disposed at one end of said envelope and serving to form an electron beam, a collector disposed at the other end of said envelope and serving to intercept said beam, a helical conductor disposed coaxially within said envelope, said cylindrical envelope including a tubular metallic portion adjacent said helical conductor, means serving to hold said helical conductor coaxially within said envelope portion, and coupling means operatively associated with each end of the helical conductor for coupling electrical energy onto and off the helical conductor, each said coupling means comprising a cylindrical metallic portion forming a seal with the elongated envelope portion, a ceramic cylinder forming a window carried at the other end of said cylindrical portion, a sealing plate carried at the outer end of the ceramic portion and forming a hermetic seal therewith, the adjacent end of the helical conductor being electrically connected to said sealing plate, and a contact flange secured to said cylindrical metallic portion.

8. A travelling wave tube including an elongated evacuatcd envelope, an electron gun disposed at one end of said envelope and serving to form an electron beam, a collector disposed at the other end of said envelope and serving to intercept said beam, a helical conductor disposed coaxially within said envelope, said cylindrical envelope including a tubular metallic portion adjacent said helical conductor, means serving to hold said helical conductor coaxially within said envelope portion, and means disposed between said electron gun and said helical conductor for coupling electrical energy onto the helical conductor comprising a transversely extending hollow cylindrical metallic portion forming a seal with the elongated envelope portion,.a transversely extending ceramic cylinder forming a window carried at the other end of said cylindrical portion, a sealing plate carried at the outer end of the ceramic portion and forming a hermetic seal therewith, and a conductor secured to said sealing plate and connected to the adjacent end of the helical conductor.

9. A travelling wave tube including an elongated evacuated envelope, an electron gun disposed at one end of said envelope and serving to form an electron beam, a collector disposed at the other end of said envelope and serving to intercept said beam, a helical conductor disposed coaxially within said envelope, said cylindrical envelope including a tubular metallic portion adjacent said helical conductor and electrically insulated therefrom, means serving to hold said helical conductor coaxially within said envelope portion, and means disposed between said collector and said helical conductor for coupling electrical energy off the helical conductor comprising a transversely extending hollow cylindrical metallic portion forming a seal with the elongated envelope portion, a transversely extending ceramic cylinder forming a window carried at the other end of said cylindrical portion, a sealing plate carried at the outer end of the ceramic portion and forming a hermetic seal therewith, and a conductor secured to said sealing plate and connected to the adjacent end of the helical conductor.

10. A travelling wave tube including an elongated evacuated envelope, an electron gun disposed at one end of said envelope and serving to project an electron beam, a collector disposed at the other end of said envelope and serving to intercept said beam, a helical conductor disposed coaxially within the envelope, means serving to hold said helical conductor coaxially within said envelope, said means comprising circumferentially spaced electrical insulating rods having their axes parallel to the longitudinal axis of the helical conductor, locating means surrounding said rods over substantially their entire length and maintaining the same at predetermined circumferential spacing, means wedgingly interposed between said locating means and said envelope portion serving to provide a tight mechanical fit between the rods, the locating means and the envelope portion whereby the helical conductor has a relatively goodheat conduction path to the envelope portion, and means for coupling electrical energy onto and off of the helical conductor comprising a cylindrical metallic portion at one end forming a seal with the elongated envelope portion adjacent each opposite end of the helical conductor, a ceramic cylinder carried at the other end of each said cylindrical portion, said cylinder providing a window for the microwave energy, a sealing plate carried at the outer end of said ceramic portion and forming a seal therewith, and a conductor secured to said sealing plate and connected to the adjacent end of the helical conductor.

11. A travelling wave tube including an elongated evacuated envelope, an electron gun disposed at one end of said envelope and serving to project an electron beam, a

collector disposed at the other end of said envelope and serving to intercept said beam, a helical conductor disposed coaxially within the envelope, said envelope having an elongated cylindrical envelope portion adjacent to said helical conductor and having a relatively high thermal conductivity, means serving to hold said helical conductor within said envelope portion, and means for coupling electrical energy onto and off the helical conductor comprising hollow cylindrical metallic portions at one end forming a seal with the elongated envelope portion and disposed, respectively, between said electron gun and the input end of said helical conductor and between said collector and the output end of the helical conductor, a ceramic cylinder carried at the other end of each cylindrical portion, a sealing plate on the end of the ceramic cylinder remote from the cylindrical metallic portion and forming a seal therewith, and a conductor secured to said sealing plate and connected to the adjacent end of the helical conductor. I

12. A travelling wave tube including an elongated evac uated envelope, an electron gun including a tubular metallic anode disposed at one end of said envelope and serving to project an electron beam, a collector disposed at the other end of said envelope and serving to intercept said beam, a helical conductor disposed coaxially within the envelope, said envelope portion adjacent to the helical conductor being a tubular metallic portion, means serving to hold said helical conductor coaxially within the tubular metallic envelope portion, said means comprising circumferentially spaced electrical insulating rods having their axes parallel to the longitudinal axis of the helical conductor, locating means surrounding said rods and maintaining the same at predetermined circumferential spacing, means interposed between said locating means and said metallic envelope portion serving to provide a tight mechanical fit between the rods, the locating means and the envelope portion whereby the helical conductor has a relatively high heat conduction path to the envelope portion, an outwardly extending flange member carried by said anode portion, an outwardly extending flange portion carried by said metallic envelope portion adjacent to said anode, and a ceramic ring secured between said flanges and serving to form a portion of the envelope and electrically insulate the electron gun from the tubular metallic portion.

13. A travelling wave tube including an elongated evacuated envelope, an electron gun including a tubular metallic anode disposed at one end of said envelope and serving to project an electron beam, a collector disposed at the other end of said envelope and serving to intercept said beam, a helical conductor disposed coaxially within the envelope, said envelope portion adjacent to the helical conductor being a tubular metallic portion, means serving to hold said helical conductor coaxially within the envelope portion, said means comprising longitudinally extending circumferentially spaced electrical insulating rods having their axes parallel to the longitudinal axis of the helical conductor, thermally conductive locating means extending longitudinally through said tubular metallic envelope portion and surrounding said rods and maintaining the same at predetermined circumferential spacing, and thermally conductive means interposed between said locating means and said envelope portion serving to provide a tight mechanical fit between the rods, the locating means and the envelope portion whereby the helical conductor has a relatively high heat conduction path to the envelope portion, a sealing ring connected to the collector, a sealing ring connected to the metallic envelope portion, and at least one ceramic ring brazed between said sealing rings and serving to form a ceramic envelope portion which electrically insulates the collector from the metallic tubular envelope portion.

14. A travelling wave tube including an elongated evacuated envelope, an electron gun including a tubular metallic anode disposed at one end of said envelope and serving to project an electron beam, a metallic collector disposed at the other end of said envelope and serving to intercept said beam, a helical conductor disposed coaxially within the envelope, said envelope portion adjacent to the helical conductor being a tubular metallic portion, means serving to hold said helical conductor coaxially within the envelope portion, said means comprising eircumferentially' spaced electrical insulating rods having their axes parallel to the longitudinal axis of the helical conductor, locating means surrounding said rods and maintaining the same at predetermined circumferential spacing, means interposed between said locating means and said envelope portion serving to provide a tight mechanical fit between the rods, the locating means and the envelope portion whereby the helical conductor has a relatively high heat conduction path to the envelope portion, an outwardly extending metallic flange secured to said anode portionan outwardly extending metallic flange secured to said tubular metallic envelope portion, a ceramic ring secured between said flanges forming part of the elongated envelope, and serving to electrically insulate the metallic envelope portion from theelectron gun, an outwardly extending flange secured to the other end of said tubular metallic envelope portion, an outwardly extending metallic flange secured to the metallic collector, and a ceramic ring secured between said flanges forming a part of the envelope and serving to electrically isolate the collector from the envelope portion.

.15. A travelling wave tube including an elongated evacuated envelope, an electron gun including a tubular metallic anode disposed at one end of said envelope and serving to project an electron beam, a collector disposed at the other end of said envelope and serving to intercept said beam, a helical conductor disposed coaxially within the envelope, said envelope portion adjacent to the helical conductor being a tubular metallic portion, means serving to hold said helical conductor coaxially within said metaliic envelope portion, an insulating cylinder disposed between said envelope portion and said anode and serving to maintain the same coaxial, an outwardly extending flange member carried by said anode portion, an outwardly extending flange portion carried by said metallic envelope 1U portion adjacent to said anode, and a ceramic ring secured between said flanges and serving to form a portion of the envelope and electrically insulat the electron gun from the tubular metallic portion.

16. A travelling wave tube including an elongated eevacuated envelope, an electron gun disposed at one end of said envelope and serving to project an electron beam, a collector disposed at the other end of said envelope and serving to intercept said beam, a helical conductor disposed coaxially within said envelope, means serving to hold said helical conductor coaxially within said envelope, said means comprising circumferentially spaced electrical insulating rods having their axes parallel to the longitudinal axis of the helical conductor, at least one tubular locating means surrounding said rods over a substantial portion of their length, said last named means including curved portions adapted to receive the associated peripheral portions of said rods, said curved means being located at predetermined circumferential spacing on said tubular member, and thermally conductive means interposed between said tubular locating means and said envelope portion to provide a tight mechanical fit between the rods, the tubular locating means, and the envelope portion whereby the helical conductor has a relatively high heat conduction path to the adjacent envelope portion.

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